Chapter 3

Elatroanalytical Methods

1 What is Voltammety? •A group of analytical methods in which only a small portion of material is electrolytically reduced or less commonly oxidized •Classification of voltammetric methods is based on:  type How the potential is applied How the current is measured

2 Characteristics

• Voltammetry is based upon the measurement of a current that develops in an electrochemical cell under conditions of complete concentration polarization. • Potentiometric measurements are made at currents that approach zero and where polarization is absent • Furthermore, in voltammetry a minimal consumption of analyte takes place, whereas in electrogravimetry and essentially all of the analyte is converted to another state • Voltammetry (particularly classical ) was an important tool used by chemists for the determination of inorganic and certain organic species in aqueous solutions.

3 Concept

Detector/ Excitation signal voltage Sample Transducer/ Readout Process Sensor

View Voltage is applied to Current is current as a analyte; appreciable transformed function of current is measured to voltage by time or electronics applied voltage Current is a function of • analyte concentration • how fast analyte moves to electrode surface • rate of electron transfer to sample

• 4 voltage, time... II. Excitation process • A. What happens when a voltage is applied to an electrode in solution containing a species? generic redox species O • O + e- --> R E = -0.500 V v. SCE • Imagine that we have a Pt electrode in sol’n at an initial potential of 0.000 V v. SCE and we switch potential to -0.700 V. • First:

 O  O  supporting O  E =0.0 Pt   electrolyte app  O  5 O O = redox solvent B. Events that happen 1. supporting electrolyte forms an electrical double layer  O   O     O  Eapp= -0.7 Pt  O O  

double layer acts as a capacitor

cation movement to electrode causes an initial spike in current Formation of double layer is good because it ensures that no electric field exists across whole sol’n (requires 100:1 conc ratio of supporting 6 elyte:redox species). 2. Electron transfer reaction

O is converted to R at electrode surface.

   O O     O R  Eapp= -0.7 Pt  O O

 O R 

{

A depletion region of O develops - a region in which conc of O is zero. How does more O get to electrode surface? mass transport mechanisms

7 C. Mass transport to the electrode

1. Migration - movement in response to electric field. We add supporting electrolyte to make analyte’s migration nearly zero. (fraction of current carried by analyte  zero) 2. Convection • stirring 3. Diffusion In experiments relying upon diffusion, no convection is desired, sol’n is quiescent.

8 Solutions and 1. Solutions: redox couple + solvent + supporting electrolyte • supporting elyte: salt that migrates and carries current, and doesn’t do redox in your potential window of interest • a wide potential window is desirable • water - good for oxidations, not reductions except on Hg supporting elytes: lots of salts • nonaqueous solvents: acetonitrile, dimethylformamide, etc.

• supporting electrolytes: tetraalkylammonium BF4, PF6, ClO4 • Oxygen is fairly easily reduced - we remove it by

deoxygenating with an inert gas (N2, Ar).

9 2. Electrodes • working etrode (WE) is where redox activity occurs • auxiliary etrode (AE) catches current flow from WE • reference etrode (RE) establishes potential of WE • a. working etrode materials: • Pt, Au, C, semiconductors • Hg - messy but good for reductions in water. Not good for oxidations. • b. auxiliary etrodes: similar materials, large in area • c. reference etrodes: real vs. quasi - • real refs have an actual redox couple (e.g. Ag/AgCl) • quasi refs (QRE) - a wire at which some (unknown) redox process occurs in sol’n. QREs OK if currents are needed but not potentials.

10 Voltammetric Techniques

• Polarography • Square Wave Voltammetry • • LSV • Differential Pulse • Normal Pulse • Sampled DC • Stripping Analysis

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13 DC voltage source

/ammeter

ammeter

Reference Cathode indicator electrode

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15 Supporting Electrolyte

• Polaragrams are recorded in the presence of a relatively high concentration of a base electrolyte such as KCI.

• The base electrolyte will decrease the resistance for the movement of the metal ions to be determined thus, the IR drop throughout the cell will be negligible.

• It helps also the movement of ions towards the electrode surface by diffusion only.

• The discharge potential of the base electrolyte takes place at a very low negative potential therefore, most ions will be reduced before the base electrolyte species. • Buffering & elimination of interferent by complexation

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18 Static Mercury Drop Electrode

19 20 Model 394 Voltammetric Analyzer Computer • controlled polarographic and voltammetric analyzer PC compatible • Windows software Can use existing • 303A / 305 21 Wide Range of Techniques

• Square Wave Voltammetry • Cyclic Voltammetry • LSV • Differential Pulse • Normal Pulse • Sampled DC • Stripping 22 Analysis Pre-experiment selection

• Analyzer consol controls SMDE • Automatic purging and stirring of sample • Automatic conditioning of electrode • Automatic control of

23 deposition times

Standards

• Up to nine standards can be entered • Selection of common potentials • Electrolyte

24 record

Multi-element Analysis

25 26 27 28 29 30 31 Advantages and Disadvantages of the Dropping Mercury Electrode

• High overvoltage associated with the reduction of hydrogen ions. As a consequence, metal ions such as zinc and cadmium can be deposited from acidic solution even though their thermodynamic potentials suggest that deposition of these metals without hydrogen formation is impossible. • A second advantage is that a new metal surface is generated continuously; thus, the behavior of the electrode is independent of its past history. In contrast, solid metal electrodes are notorious for their irregular behavior, which is related to adsorbed or deposited impurities. • A third unusual feature of the dropping electrode, which has already been described, is that reproducible average currents are immediately realized at any given potential whether this potential is approached from lower or higher settings.

32 • One serious limitation of the dropping electrode is the ease with which mercury is oxidized: this property severely limits the use of the electrode as an anode. At • potentials greater than about + 0.4 V, formation of mercury(I) gives a wave that masks the curves of other oxidizable species. • In the presence of ions that form precipitates or complexes with mercury(I), this behavior occurs at even lower potentials. For example, in the Figure, the beginning of an anodic wave can be seen at 0 V due to the reaction - - • 2Hg + 2CI < === > Hg2CI2(s) + 2e • Incidentally, this anodic wave can be used for the 33 determination of chloride . • Fig. 18: Residual current curve for a 0.1M solution of HCl

34 • Another important disadvantage of the dropping mercury electrode is the nonfaradaic residual or charging current, which limits the sensitivity of the classical method to concentrations of about 10- 5 M. • At lower concentrations, the residual current is likely to be greater than the diffusion current, a situation that prohibits accurate measurement of the latter. • Finally, the dropping mercury electrode is cumbersome to use and tends to malfunction as a result of clogging.

35 Effect of Dissolved Oxygen

• Oxygen dissolved in the solution will be reduced at the DME leading to two well defined waves which were attributed to the following reactions: + - O2(g) + 2H + 2e < ==== > H2O2 + - H2O2 + 2H +2e < ==== > 2H2 O

• E1/2 values for these reductions in acid solution correspond to -0.05V and -0.8V versus SCE .

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42 Polarogram for (A) 5X10-4M Cd2+ in 1M HCl. (B) 1M HCl

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708

1 / 2 2 / 3 1/6 Id (average) = 6/7 (708) n D m t C 45

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51 Currents controlled by factors other than diffusion • Processes other than diffusion are involved on the electrode surface: 1. Chemical reactions involving oxidation or reduction 2. Adsorption of electroactive species

52 Kinetic Currents

• Currents whose magnitudes are controlled by that rates of chemical reactions A (not electroactive) + X Ox

k A + X 1 Ox + ne R

CH2O(H2O) CH2O + H2O + CH2O + 2H + 2e CH3OH il = id + ik 53 Catalytic Current

• It is controlled by a catalytic process • Either the electroactive substance is regenerated by a chemical reaction 3+ 2+ 3+ Fe + e Fe + H2O2  Fe • The electroreduction of a species is shifted to a more +ve potential • Proteins catalyze the reduction of H+ and shift the corresponding wave to a more +ve potential

• ik is a nonlinear function of concentration or linear over a limited concentration range

54 Adsorption Currents

• If oxidized form is adsorbed its reduction will take place at a more –ve potential than the diffusion current

• If reduced form (product) is adsorbed its reduction will take place at a more +ve (prior) potential than the diffusion current

55 Polarographic Maxima

• Currents that are at a certain point of potential higher (about 2 order of magnitude) than the diffusion current • Be removed by addition of surfactant (triton-100) or gelatin

56 Tests of Current Limiting Processes

• Usually the currents are distinguished from each other by the changes that take place when the following parameters are varied: • Concentration of electroactive species • Mercury column height • pH • Buffer concentration • Temperature

57 58 59 60 Polarographic wave shapes

Consider the following reversible equilibrium reaction at the electrode surface :

A + ne B

61 since [A]o is the difference between the amount of A that was initially at the electrode (an amount that would produce the limiting current, id, if entirely reduced) and the amount remaining after the formation of [B]o. By analogy to the constants in the Ilkovic equation, the proportionality constants k and k' are identical except for the diffusion coefficients of A and B, and so Equation 3.10 becomes

62 63 64 • Equation 3.15 holds for reversible, diffusion-controlled electrochemical reactions where the product is initially absent in the bulk solution, and is soluble in the solution or in the electrode itself (as an amalgam, which is the case for reduction of many metal ions).

• A plot of Eapplied versus log[i/(id - i)] can be used as a test for these conditions (a straight line would be obtained).

• It is also a means of determining n (from the slope) and E1/2. • The interpretation requires that in addition to a straight line, a reasonable, integral n-value be obtained before reversibility can be claimed.

• The E1/2-value is useful because it provides an estimate of E°'; 1/2 the term log(DA/DB) is generally small. • If the electrode reaction is not reversible, the rising portion of the polarographic wave is drawn out. This occurs when the rate constants near E°' are too small to allow equilibrium to be reached on the time scale of the experiment. 65 Effect of complex formation on polarographic waves

• When the metal ion forms a complex with a

ligand, a shift in the E1/2 takes place. This shift goes towards more –ve potential • The the magnitude of this shift is proportional to the stability of the complex as well as to the concentration of the ligand. • Formation constants can be estimated from the

magnitude of the shift in the E1/2

66 • Previous equation (3.15) is not applicable if either species A or B is adsorbed on the electrode or is involved in a chemical reaction other than simple electron transfer. • A common example of the latter is the case where A is a metal ion that is in equilibrium with p molecules of a ligand, L, and a metal complex, ALP:

67 • When the term [A]o in Equation 3.10 is replaced by [ALP]o/Kf[L]o, and, if the concentration of the ligand is in large excess over that of A, the following expression can be written

68 • The slope of a plot of (E1l2)coplx versus log [L] yields "p" (if n is known)

• and the intercept, where [L] = 1 M, can be used to

calculate Kf if (E1/2)free ion is known.

69 Analytical Applications

• Direct calibration method (external standards method) • Standard addition method • Internal standard method • Examples of the electroactive species and applications can be found in the book p. 67-76

70 71 In the method of standard additions, a known amount of analyte is added to the unknown. The increase in signal intensity tells us how much analyte was present prior to the standard addition.

ld(unknown) = kCx where k is a constant of proportionality. Let the concentration of standard solution be CS. When VS mL of standard solution is added to Vx mL of unknown, the diffusion current is the sum of diffusion currents due to the unknown and the standard.

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The contribution of the charging current will be minimized and the spikes will disappear leading to a smoother polarogram ( stair- shape polarogram ). 77 Pulse Polarography

• DC: icha almost equal ifar

• in ifar/ icha ratio • Change in the electrode area is very rapid in early stages and almost constant close to the end • In pp the potential will not be applied until the area-time curve is flattened out

• ifar and icha decay in time but the decay of

ifar is much slower 1. Normal Pulse polarog. : gradual increase in the amplitude in the voltage pulse 2. Differential pulse polarog.: Voltage pulse of constant amplitude superimposed on a slowly increasing voltage

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Series of pulses (40 ms duration) of increasing amplitude (potential) are applied to successive drops at a preselected time (60 ms) near the end of each drop lifetime. Between the pulses, the electrode is kept at a constant base potential where no reaction occurs 80 • ic is very large at the beginning of the pulse; it then decays exponentially. • i is measured during the 20 ms of the second half of the pulse when ic is quite small • The current is sampled once during each drop life and stored until next sample period, thus the polarogram shows a staircase appearance • NPP is designed to block electrolysis prior to the measurement period

81 Differential Pulse Polarography

• A pulse (of constant amplitude of 5-100 mV) of 40-60 ms is applied during the last quarter of the drop life • The pulse is superimposed on a slowly increasing linear voltage ramp. • The current is measured twice: one immediately preceding the pulse and the other near the end of the pulse. • Overall response plotted is the difference between the two currents sampled

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Fixed magnitude pulses (50 mV each) superimposed on a linear potential ramp are applied to the working electrode at a time just before the drop falls (last 50 ms). The current is measured at 16.7 ms prior to the DC pulse and 16.7 ms before the end of the pulse.

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84 85 Voltamunogram for a differential pulse polarography experiment

86 87 88 (a) Differential pulse polarogram: 0.36 ppm tetracycline- HCI in 0.1 acetate buffer, pH 4, PAR Model 174 polarographic analyzer, dropping mercury electrode, 50-mV pulse amplitude, 1-s drop. 89 (b) DC polarogram: 180 ppm tetracycline · HCI in 0.1 M acetate buffer, pH 4, similar conditions. Differential pulse polarogram

The example above shows the simultaneous determination of Zn , 90 Cd, Pb and Cu using standard addition Applications •Determination of trace elements Pb, Cd, Cu, Fe, Ni, Co, Al, Cr, Hg .... •Determination of nitrate, nitrite, chloride, iodiide, cyanide, oxygen ..... •Determination of numerous organic and toxic materials - surfactants, herbicides, pesticides, insecticides, nitro compounds, halogenous compounds

91 Detection Limits in the ultra trace range

3 0.05 0.05 0.05 0.1 0.03 1 Al Cu Pb Zn As Fe3+ Cl ppb ppb ppb ppb ppb ppm ppb

1 0.03 0.05 0.1 0.5 0.05 PO4 20 Ag Hg Mo As3+ Mn NO3- ppb ppb ppb ppb ppb ppm 2- ppm

0.05 0.01 0.01 0.01 0.03 0.01 SO4 20 Cd Cr4+ Pd Co Fe NO2- ppb ppb ppb ppb ppm ppm 2- ppb

0.01 0.01 1 0.05 0.1 NH4 60 0.01 Cr Ni Se Ti Be S2- ppb ppb ppb ppb ppb + ppm ppm

92 93 94 95 96 97 98 99 100 Linear Scan Voltammetry

•The whole potential (0 – 2 V or any other pot. range) is carried out on a constant electrode surface; one Hg drop in a rapid manner, 50 to 50,000 mV/sec (Fast Linear scan/sweep voltammetry) •In polarog. i-E relationship is independent of scan rate since the voltage is almost constant during the drop life;it is in the order of mV or less. •Mechanistic studies require the high end of scan rates and analytical studies require the slower end •Hanging mercury drop electrode is used. Pt, graphite or others could be used when more +ve potential is required

101 •As the potential goes more –Ve below Eo an increase in the electeolytic process takes place until th surface concentration drops to almost zero and the rate of transfer of the electroactive species reahces max. A peaked i-E is then observed. •As the potential goes more -Ve than Eo a depletion of the electroactive species in the electrode viscinity takes place. The rate of replenishment is slow as a result of the fast scan rate. The surface concnetration and the current will then decrease. •Randlis-Sevcik Eq.: 3/2 1/2 1/2 ip = kn AD C v 1/2 •ip  v and ic  v. Thus ip/ ic ratio decreses with scan rte

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106 Anodic

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Pb Cu Cd

+ 0.3 V -1 V

108 Example of ASV: Determination of Pb at HDME • Deposition (cathodic) reduce Pb2+ Eapp – Stir (maximize convection) I • Concentrate analyte • Stop stirring = Ip equilibration/rest period • Scan E in anodic sense Pb  Pb2+ + 2e- and record voltammogram – oxidize analyte (so redissolution occurs)

109 Applications of HDME ASV

• Usually study M with Eo more negative than Hg – EX: Cd2+, Cu2+, Zn2+, Pb2+ • Study M with Eo more positive than Hg at Glassy carbon electrode – EX: Ag+, Au+, Hg • Can analyze mixture with Eo  100 mV

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111 Anodic Stripping Voltammetry Differential-pulse anodic stripping voltammogram of 25 ppm zinc, cadmium, lead, and copper. 112 Cathodic Stripping voltammetry

• Anodic deposition –Form insoluble, oxidized Hg salt of analyte anion –Stir (maximize convection) • Equilibrate (stop stirring) • Scan potential in opposite sense (cathodic) –Reducing salt/film and forming 113soluble anion • Record voltammogram 114 115 116 117 Applications of HDME CSV

• Can study halides, sulfides, selenides, cyanides, molybdates, vanadates

• EX: FDA 1982-1986 used to confirm CN- (-0.1 V) in Tylenol Crisis

118 119 Sensitivity of Voltammetry

• Voltammetric techniques are ranked amongst one of the most sensitive analytical techniques. • Concentrations of certain metals can be determined at sub-part per billion level. • Many trace and ultra-trace organic determinations can be conveniently

120made. Speed of Voltammetric Techniques • Analysis using • Polarographic or multiple electrodes voltammetric possible aalyzer consol • Fast techniques controls complete such as square process of analysis. wave voltammetry

possible • For liquid and gaseous samples dilution in appropriate liquid may be sufficient 121 Multi-component capability

• Simultaneous determination of several analytes by a single scan. • Voltammetry can determine metals, organics and anions in one procedure.

122 Real Benefits • Conventional methods of analysis may require long, involved preparative techniques to concentrate the species of interest or remove contaminating or interfering ions. • These preparations risk contaminating the sample. Polarography and voltammetry can offer a more effective, realible tool for speciation analysis of natural water where the analyte of interest is in the sub ppm range. • Without the long preparation you'll have more free time 123 DC Polarography DC Stripping Voltammetry Adsorptive Stripping Voltammetry Differential Pulse Polarography Cyclic Voltammetry

The sensitivity of the instrument is comparable with AAS and in many cases it is even better.

124 Amperometric Titrations

• il is mesured during the titration to detect the end point

• il is proportional to concnetration and can be detected due to titrant, analyte, product or any two of them

1. Using one Polarized or Indicator Elctrode

2+ 2- Example: Titration of Pb versus SO4 • Lead is reducible at about –0.4 V but sulfate is not

• Constant voltage (-1 V)that has any value on the id-E curve is applied • The id-volume of titant curve will depend upon the electoactivity

of125 the species 126 Two Polarized or Indicator Electrodes Bi-amperometric Titrations

•Asmall fixed potential difference (20-250 m?V)is applied across two identical (Pt) indicator electrodes. The current is measured during the titration * Example: Titration of Fe2+ vs Ce4+

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